Whereas the risk of arteriosclerosis increases with increasing levels of low density lipoproteins in plasma, risk is inversely related to levels of high density lipoproteins (HDL). The mechanisms by which HDL could exert a protective effect are not yet established. Therefore much more needs to be known about their structure and function. The traditional means of isolation of HDL, ultracentrifugation, has been shown to obscure their molecular speciation and biological functions. In contrast, isolation by selective affinity immunosorption preserves both the molecular architecture and function of HDL species. This project will focus on isolating and characterizing the native species of HDL using selected affinity immunosorption. The architectures of the individual species will be determined. Several newly discovered HDL proteins will be cloned and sequenced. The function of newly isolated species will be studied with emphasis on their ability to participate in the retrieval of cholesterol from cells, their antioxidant properties and their physical interaction with proteoglycans found in the artery wall. Particular attention is given to study of a newly emerging HDL species, the 67 kDa prebeta-1 HDL, a particle that appears to play a key role in the retrieval of cholesterol from peripheral tissues. Its possible formation during lipolysis and during the selective uptake of cholesteryl esters by the newly discovered SR-BI receptor will be explored and its behavior in disorders of lipid metabolism will be studied. Knowledge of the molecular speciation of HDL could lead to new strategies of prevention and treatment of arteriosclerotic heart disease and may provide insights for understanding other roles of HDL in biology.